For Each loop: Some items get skipped when looping through Outlook mailbox to delete items - vba

Question

I wanted to develop VBA code that:
Loops through all email items in mailbox
If there are any type of other items say "Calendar Invitation" skips that item.
Finds out the emails with attachments
If attached file has ".xml" extension and a specific title in it, saves it to a directory, if not it keeps searching
Puts all email includes .xml attachments to "Deleted Items" folder after doing step 4 and deletes all emails in that folder by looping.
Code works perfect EXCEPT;
For example
There are 8 email received with ".xml" file attached to each one of them in your mailbox.
run the code
you will see only 4 of the 8 items are processed successfully, other 4 remain in their positions.
If you run the code again, now there would be 2 items processed successfully and other 2 remain in your mailbox.
Problem: After running the code, it is supposed to process all files and deletes them all not the half of them in each run. I want it to process all items at a single run.
BTW, this code runs every time I open the Outlook.
Private Sub Application_Startup()
'Initializing Application_Startup forces the macros to be accessible from other offic apps
'Process XML emails
Dim InboxMsg As Object
Dim DeletedItems As Outlook.Folder
Dim MsgAttachment As Outlook.Attachment
Dim ns As Outlook.NameSpace
Dim Inbox As Outlook.Folder
Dim fPathTemp As String
Dim fPathXML_SEM As String
Dim fPathEmail_SEM As String
Dim i As Long
Dim xmlDoc As New MSXML2.DOMDocument60
Dim xmlTitle As MSXML2.IXMLDOMNode
Dim xmlSupNum As MSXML2.IXMLDOMNode
'Specify the folder where the attachments will be saved
fPathTemp = "some directory, doesn't matter"
fPathXML_SEM = "some directory, doesn't matter"
fPathEmail_SEM = "some directory, doesn't matter"
'Setup Outlook
Set ns = GetNamespace("MAPI")
Set Inbox = ns.Folders.Item("mailbox-name").Folders("Inbox")
Set DeletedItems = ns.Folders.Item("mailbox-name").Folders("Deleted Items")
'Loop through all Items in Inbox, find the xml attachements and process if they are the matching reponses
'On Error Resume Next
For Each InboxMsg In Inbox.Items
If InboxMsg.Class = olMail Then 'if it is a mail item
'Check for xml attachement
For Each MsgAttachment In InboxMsg.Attachments
If Right(MsgAttachment.DisplayName, 3) = "xml" Then
'Load XML and test for the title of the file
MsgAttachment.SaveAsFile fPathTemp & MsgAttachment.FileName
xmlDoc.Load fPathTemp & MsgAttachment.FileName
Set xmlTitle = xmlDoc.SelectSingleNode("//title")
Select Case xmlTitle.Text
Case "specific title"
'Get supplier number
Set xmlSupNum = xmlDoc.SelectSingleNode("//supplierNum")
'Save the XML to the correct folder
MsgAttachment.SaveAsFile fPathXML_SEM & xmlSupNum.Text & "_" & Format(Date, "yyyy-mm-dd") & ".xml"
'Save the email to the correct folder
InboxMsg.SaveAs fPathEmail_SEM & xmlSupNum.Text & "_" & Format(Date, "yyyy-mm-dd") & ".msg"
'Delete the message
InboxMsg.Move DeletedItems
Case Else
End Select
'Delete the temp file
On Error Resume Next
Kill fPathTemp & MsgAttachment.FileName
On Error GoTo 0
'Unload xmldoc
Set xmlDoc = Nothing
Set xmlTitle = Nothing
Set xmlSupNum = Nothing
End If
Next
End If
Next
'Loop through deleted items and delete
For Each InboxMsg In DeletedItems.Items
InboxMsg.Delete
Next
'Clean-up
Set InboxMsg = Nothing
Set DeletedItems = Nothing
Set MsgAttachment = Nothing
Set ns = Nothing
Set Inbox = Nothing
i = 0
End Sub

Answer 1

Likely cause: When you do this InboxMsg.Move, all of the messages in your inbox after the one that was moved are bumped up by one position in the list. So you end up skipping some of them. This is a major annoyance with VBA's For Each construct (and it doesn't seem to be consistent either).
Likely solution: Replace
For Each InboxMsg In Inbox.Items
with
For i = Inbox.Items.Count To 1 Step -1 'Iterates from the end backwards
Set InboxMsg = Inbox.Items(i)
This way you iterate backward from the end of the list. When you move a message to deleted items, then it doesn't matter when the following items in the list are bumped up by one, because you've already processed them anyway.

Answer 2

It's often not a good idea to modify the contents of a (sub)set of items while looping over them. You could modify your code so that it first identifies all of the items that need to be processed, and adds them to a Collection. Then process all the items in that collection.
Basically you shouldn't be removing items from the Inbox while you're looping through its contents. First collect all the items you want to process (in your Inbox loop), then when you're done looping, process that collection of items.
Here's some pseudo-code which demonstrates this:
Private Sub Application_Startup()
Dim collItems As New Collection
'Start by identifying messages of interest and add them to a collection
For Each InboxMsg In Inbox.Items
If InboxMsg.Class = olMail Then 'if it is a mail item
For Each MsgAttachment In InboxMsg.Attachments
If Right(MsgAttachment.DisplayName, 3) = "xml" Then
collItems.Add InboxMsg
Exit For
End If
Next
End If
Next
'now deal with the identified messages
For Each InboxMsg In collItems
ProcessMessage InboxMsg
Next InboxMsg
'Loop through deleted items and delete
For Each InboxMsg In DeletedItems.Items
InboxMsg.Delete
Next
End Sub
Sub ProcessMessage(InboxMsg As Object)
'deal with attachment(s) and delete message
End Sub